The world's power consumption is sharply increasing mainly due to the rapid industrial developments and the improved standards of living. Renewable power sources become more important through the increasing demand for power. Renewable power sources such as photovoltaic systems or wind farms represent a plurality of power sources that feed power into the public power grid.
There exist various types of power generation systems. There are common power generation systems that exclusively produce and provide power for the public power grid. Besides such large power generation systems there are local and smaller power generation systems that produce power for example for a premises. Common power generation systems with a renewable power source usually comprise an inverter that converts a produced direct current (DC) power to single or multiphase alternating current (AC) output power. From an output of the inverter the output power is provided to a local electrical load. For example the power consumption of a building may represent such a local load. The power output of the inverter is often connected additionally to the public power grid. If the power demand of the local load is less than the power provided from the power source the surplus power is fed into the public power grid. In case the local demand exceeds the power provided from the renewable power source, the missing power is obtained from the public power grid.
As the power production as well as the power consumption of the local load may vary over time often an energy storage device such as a battery is installed in order to temporarily store energy. The storage systems usually comprise a controller that controls the inverter. Based on the measured power demand and according to the present available power from the renewable power source the controller controls the inverter to charge or to discharge the storage device in order to meet the present power demand. Surplus power from the power source can be stored in the storage device. Exceeds the local power consumption the produced power additional power can be obtained from the storage device.
The ratio of the consumed power from the renewable energy source to the total of the produced power from the renewable energy source indicates the self-consumption ratio (SCR). Whereas the autonomy ratio (AR) indicates the ratio of the self-produced energy to the energy obtained from the grid.
Various types of controllers are known in the art that control the outgoing power or current of an inverter of a renewable power source in order to meet output power specifications or to meet public power grid requirements.
For example US 2013/0181527 A1 (Sinewatt Inc.) describes a solar photovoltaic energy collection and conversion system. The system has several string members that include a photovoltaic power source and a controller. The controller coordinates the output of each string member to achieve a desired power based on the performance or operation of each string member. The system includes converting the DC power to AC power, sharing or equalizing of the current from a string member to other string members or synchronizing the current generation among the string members in order to synchronize the current with the existing electrical grid. The strings may be arranged to form a single phase voltage output or three single phase strings may be grouped together electrically for a three-phase system.
A method for predictive phase balancing is disclosed in US 2014/0031998 (IBM). From receiving customer demand profiles the method creates a demand forecast from anticipated power demands. The method then breaks down loads for each customer by corresponding phases in a poly-phase power grid network so that the loads correspond to the anticipated power demands. The described system includes a power supplier with three circuit conductors that produce power in a three-phase process. A logic is configured to utilize a pre-defined maximum deviation or difference among the phases at a given time interval. The logic incrementally removes the smallest load from the phase identified as having the highest value and re-allocates the smallest load to the phase having the lowest value. A load is considered to be balanced when the load values are substantially equal.
U.S. Pat. No. 8,344,550 B2 (General Electric Comp.) discloses a power generation system with a renewable power source. The system comprises a source side converter, a source side controller, a grid side converter, a grid side controller, an electrical energy storage device and an energy storage controller. The grid side converter comprises a DC to AC inverter. The source and grid side converters comprise single phase or multi-phase configurations. The grid side controller receives grid feedback for use in providing grid switching signals to the grid side converter. Based on these signals the grid side converter adjusts the output power. That allows achieving grid interconnection requirements. For example energy that would have gone to grid can be absorbed in the energy storage device (battery). The energy storage controller controls the energy storage converter to achieve a desired power balance on the DC link. The power balance is achieved by power exchange between DC Link and the energy storage device.
The above mentioned controllers control the output power but they only consider the power grid requirements but not a local load for the controlling of the output power. There are controllers known in the art that control the output power provided to the public power grid to zero. That means the local load consumes more power from the renewable power source. Such controllers have the disadvantage that only the sum of a three-phase AC is controlled to zero. As the load often differs from one phase to another phase currents are not equal to zero. With a voltage being present at the output that means that some phases might feed power into the power grid. This unbalanced power feed in may cause grid disturbances. Further it is more efficient to consume the produced power from the power source by a local load than to feed it in the power grid.